/**
 * Mar 19, 2016 7:01:35 PM
 * PrjName: BC76-01
 * @semprathlon
 */
import java.io.*;
import java.util.*;
public class Main {
    static Map<Integer, Integer> mp=new HashMap<Integer,Integer>();
    public static void main(String[] args) throws IOException{
        InputReader in=new InputReader(System.in);
        PrintWriter out=new PrintWriter(System.out);
        int T=in.nextInt();
        while(T-->0){
            int n=in.nextInt();
            mp.clear();
            for(int i=1;i<=n;i++){
                int k=in.nextInt();
                int s=mp.containsKey(k)?mp.get(k):0;
                mp.put(k, s+1);
            }
            double ans=0;
            int tmp=n;
            for(Map.Entry<Integer, Integer> ent:mp.entrySet()){
                int s=ent.getValue();
                tmp-=s;
                if (tmp>0)
                    ans+=s/(double)n*tmp/(double)(n-1);
            }
            out.println(String.format("%.6f", ans));
        }
        out.flush();
        out.close();
    }

}
class InputReader {
    public BufferedReader reader;
    public StringTokenizer tokenizer;

    public InputReader(InputStream stream) {
        reader = new BufferedReader(new InputStreamReader(stream), 32768);
        tokenizer = null;
    }

    public String nextLine() {
        String tmp = null;
        try {
            tmp = reader.readLine();
            tokenizer = new StringTokenizer(tmp);
        } catch (IOException e) {
            throw new RuntimeException(e);
        } catch (NullPointerException e) {
            return null;
        }
        return tmp;
    }

    public String next() {
        while (tokenizer == null || !tokenizer.hasMoreTokens()) {
            try {
                tokenizer = new StringTokenizer(reader.readLine());
            } catch (IOException e) {
                throw new RuntimeException(e);
            }
        }
        return tokenizer.nextToken();
    }

    public int nextInt() {
        return Integer.parseInt(next());
    }

    public long nextLong() {
        return Long.parseLong(next());
    }

    public double nextDouble() {
        return Double.parseDouble(next());
    }
}